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(NPR)   Neil deGrasse Tyson helps unravel the mystery of the levitating Slinky   (npr.org) divider line 36
    More: Followup, Neil deGrasse, Hayden Planetarium  
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3940 clicks; posted to Geek » on 11 Sep 2012 at 4:02 PM (2 years ago)   |  Favorite    |   share:  Share on Twitter share via Email Share on Facebook   more»



36 Comments   (+0 »)
   

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2012-09-11 03:46:24 PM  
Wow, that is rather cool.  Especially the tennis ball one.
 
2012-09-11 03:53:56 PM  
Yay high school physics.
 
2012-09-11 04:03:44 PM  

ThatGuyGreg: Yay high school physics.


Party pooper.
 
2012-09-11 04:07:37 PM  
Uh...doesn't the bottom of the slinky not "fall" because the slinky is trying to contract, and the bottom is being "pulled" up at the same time that gravity is pulling it down?
 
2012-09-11 04:08:22 PM  
...and no, I didn't watch the videos - Youtube blocked at work.
 
2012-09-11 04:09:44 PM  

DontMakeMeComeBackThere: Uh...doesn't the bottom of the slinky not "fall" because the slinky is trying to contract, and the bottom is being "pulled" up at the same time that gravity is pulling it down?


Yes. But its also interesting to think of it as change propagating through the medium. The change of you releasing it and reducing the stress it doesn't instantly all destress it has to propagate.
 
2012-09-11 04:14:41 PM  
Whar is Neil? In the vids?
 
2012-09-11 04:17:34 PM  
I love science. That is all.
 
2012-09-11 04:23:25 PM  

MindStalker: DontMakeMeComeBackThere: Uh...doesn't the bottom of the slinky not "fall" because the slinky is trying to contract, and the bottom is being "pulled" up at the same time that gravity is pulling it down?

Yes. But its also interesting to think of it as change propagating through the medium. The change of you releasing it and reducing the stress it doesn't instantly all destress it has to propagate.


The article does try to make it sound more mystical than it really is though. The force that propagates isn't gravity -- that's constant -- but rather the change in tension of the slinky. Saying the bottom "doesn't know it's supposed to fall" isn't very accurate even from an analogy standpoint.
 
2012-09-11 04:29:24 PM  

DontMakeMeComeBackThere: Uh...doesn't the bottom of the slinky not "fall" because the slinky is trying to contract, and the bottom is being "pulled" up at the same time that gravity is pulling it down?


Yes. And a hanging slinky is in perfect equilibrium between gravity and tension.

Still looks cool though.
 
2012-09-11 04:34:01 PM  

imgod2u: The article does try to make it sound more mystical than it really is though. The force that propagates isn't gravity -- that's constant -- but rather the change in tension of the slinky. Saying the bottom "doesn't know it's supposed to fall" isn't very accurate even from an analogy standpoint.


I kind of cringed when they explained it. Reminds me of something on Bad Science Link
 
2012-09-11 04:34:26 PM  
it's almost as if the force of gravity pulling the slinky down causing it to stretch is equal to the force exerted when the tension is allowed to pull the slinky back into shape.
 
2012-09-11 05:14:50 PM  

SuperT: it's almost as if the force of gravity pulling the slinky down causing it to stretch is equal to the force exerted when the tension is allowed to pull the slinky back into shape.



Its kinda wild how that's the exact same force, as the bottom of the slinky (and tennis ball) both stay perfectly stationary.
 
If what you are saying is true, why wouldn't the slinky move slightly up or down based on which force is stronger?
 
/Serious question
 
2012-09-11 05:34:11 PM  
Anyone who has watched looney tunes knows that gravity has no effect on you till you look down
 
2012-09-11 05:36:19 PM  
i.imgur.com
 
2012-09-11 05:37:27 PM  

downstairs: SuperT: it's almost as if the force of gravity pulling the slinky down causing it to stretch is equal to the force exerted when the tension is allowed to pull the slinky back into shape.


Its kinda wild how that's the exact same force, as the bottom of the slinky (and tennis ball) both stay perfectly stationary.
 
If what you are saying is true, why wouldn't the slinky move slightly up or down based on which force is stronger?
 
/Serious question


If gravity was stronger than the tension, the slinky's bottom would've been moving down when it was being held. If the tension in the slinky were stronger than gravity, the slinky would've been moving up. The slinky did oscillate through this (go down and bounce back up) initially. But then it eventually settled down to equilibrium where the gravitational force was exactly equal to the tension of the slinky.
 
2012-09-11 05:37:58 PM  

downstairs: SuperT: it's almost as if the force of gravity pulling the slinky down causing it to stretch is equal to the force exerted when the tension is allowed to pull the slinky back into shape.


Its kinda wild how that's the exact same force, as the bottom of the slinky (and tennis ball) both stay perfectly stationary.
 
If what you are saying is true, why wouldn't the slinky move slightly up or down based on which force is stronger?
 
/Serious question



Treating this as a serious question: the sprung mass of slinky is pulled groundward by gravity, thereby stretching it out as it counters this force by deforming (note the narrowing gap between coils as you observe lower points on the sprung slinky). At its release, the tension in the slinky and the force of gravity are equal, so the bottom pulls up at the same rate it drops. Adding a tennis ball stretches the slinky further, as depicted in the second video in tFA, but doesn't change the mechanics.

In a previous thread, there was a link to a video of a nifty finite-element model, where the guy plotted the slinkly's centre of mass as it collapsed, which dropped at exactly the rate you'd expect for a dropped, say, golf ball, while the bottom remained visually stationary.
 
2012-09-11 05:44:22 PM  
This whole 'magic slinky' thing reminds me of the old HS physics 'trick' question:

"You tie a rope to a tree to practice for tug-of-war, and pull with all your might (200 N). Later, you compete against a rival, with each of you applying 200 N to your respective ends of the rope. How much more tension is the rope under during competition than in practice?"

That one's obvious, but the slinky thing works on the same part of the brain: the one that says "dropped things should fall" (or "400 is more than 200")
 
2012-09-11 05:51:56 PM  

impaler: imgod2u: The article does try to make it sound more mystical than it really is though. The force that propagates isn't gravity -- that's constant -- but rather the change in tension of the slinky. Saying the bottom "doesn't know it's supposed to fall" isn't very accurate even from an analogy standpoint.

I kind of cringed when they explained it. Reminds me of something on Bad Science Link


Well, TFA does only claim to be a "sciencey blog", not a science blog.
 
2012-09-11 05:58:34 PM  

Loomy: In a previous thread, there was a link to a video of a nifty finite-element model, where the guy plotted the slinkly's centre of mass as it collapsed, which dropped at exactly the rate you'd expect for a dropped, say, golf ball, while the bottom remained visually stationary.



Ok, now this makes perfect sense.  Thanks for adding that.
 
2012-09-11 06:52:59 PM  
As a physics groupie, I need to thank all the physics-inclined farkers for explaining it. Now it all makes sense.

/Serious thanks
 
2012-09-11 08:00:41 PM  

gittlebass: Anyone who has watched looney tunes knows that gravity has no effect on you till you look down


And here I had this image URL all ready to go :)

2.bp.blogspot.com
 
2012-09-11 08:25:14 PM  
What if you do this on a moving vehicle?

What if you do this on an ascending airplane?

What if you do this on an airplane taking off from a conveyor belt runway?
 
2012-09-11 08:42:31 PM  

LordOfThePings: What if you do this on a moving vehicle?

What if you do this on an ascending airplane?

What if you do this on an airplane taking off from a conveyor belt runway?


This always happen anytime there's motion; including cars slowing down, planes taking off, etc.

It's just that those objects are far more rigid so you don't notice the tiny amounts of compression or stretch like you would with a slinky.
 
2012-09-11 09:05:41 PM  

imgod2u: This always happen anytime there's motion; including cars slowing down, planes taking off, etc.

It's just that those objects are far more rigid so you don't notice the tiny amounts of compression or stretch like you would with a slinky.


No, dropping a slinky in the given situation. I know, all you're doing is adding momentum to the mix.

(Was being facetious; the airplane conveyor was a more interesting thought experiment to me than the tennis ball.)
 
2012-09-11 09:16:30 PM  

I'm an Egyptian!: As a physics groupie, I need to thank all the physics-inclined farkers for explaining it. Now it all makes sense.

/Serious thanks


FARK: Where any measure of politeness must have sarcasm disclaimed.
 
2012-09-11 09:22:43 PM  

Loomy: downstairs: SuperT: it's almost as if the force of gravity pulling the slinky down causing it to stretch is equal to the force exerted when the tension is allowed to pull the slinky back into shape.


Its kinda wild how that's the exact same force, as the bottom of the slinky (and tennis ball) both stay perfectly stationary.
 
If what you are saying is true, why wouldn't the slinky move slightly up or down based on which force is stronger?
 
/Serious question


Treating this as a serious question: the sprung mass of slinky is pulled groundward by gravity, thereby stretching it out as it counters this force by deforming (note the narrowing gap between coils as you observe lower points on the sprung slinky). At its release, the tension in the slinky and the force of gravity are equal, so the bottom pulls up at the same rate it drops. Adding a tennis ball stretches the slinky further, as depicted in the second video in tFA, but doesn't change the mechanics.

In a previous thread, there was a link to a video of a nifty finite-element model, where the guy plotted the slinky's centre of mass as it collapsed, which dropped at exactly the rate you'd expect for a dropped, say, golf ball, while the bottom remained visually stationary.


Done in one Nth post.

/Solve for N.
//cool vid.
 
2012-09-11 09:23:46 PM  

LordOfThePings: I know, all you're doing is adding momentum to the mix.


In a static evaluation you're not even "adding" momentum, you're just redefining the frame of reference for velocity.
 
2012-09-12 01:24:39 AM  
DontMakeMeComeBackThere: Uh...doesn't the bottom of the slinky not "fall" because the slinky is trying to contract, and the bottom is being "pulled" up at the same time that gravity is pulling it down?

I think you got the basics of it. The whole slinky is retracting AND falling. The bottom accelerates upward at the same rate gravity accelerates the whole slinky downwards. I'd be willing to bet the top accelerates downward faster: at the rate of gravity plus the rate of contraction.
 
2012-09-12 01:33:33 AM  
 
2012-09-12 05:19:50 AM  

LordOfThePings: What if you do this on a moving vehicle?


They did. The Earth is moving.

/Mind blown
 
2012-09-12 10:01:36 AM  

Horatio Noseblower: I'd be willing to bet the top accelerates downward faster: at the rate of gravity plus the rate of contraction.


And you would be correct. Intuitive proof: the whole slinky must be falling under the influence of gravity, since there is no magic here. Therefore the slinky's center of mass must be falling in a perfect parabolic curve (plotting position vs. time). Since the bottom of the slinky is not moving, the top must be moving faster than gravity alone would expect; otherwise the center of mass would not be falling fast enough.
 
2012-09-12 10:07:36 AM  
Also, I really dislike the "doesn't know" explanation given in the article. There are force acting on the various parts of the slinky, and one could readily diagram them and explain them in terms of forces. Or even just explain that, when dropped, the forces balance at the bottom of the slinky but not at the top of the slinky (again this is somewhat intuitive; before the drop, the person holding the top of the slinky is exerting a direct force on the top of the slinky, but only indirectly on the bottom).
 
2012-09-12 12:01:14 PM  
Ok, hypothetical. Let's say that Nero had beamed Vulcan's sun away some sufficient distance-- bam, fizzle, gone-- instead of sucking Vulcan into a black hole. Let's also suppose that Vulcan is the same distance from its sun as Earth is from Sol. Sun leaves, taking its gravitational pull with it. Does this principle of propagation mean that it would take 8 minutes after its sun's disappearance for Vulcan to be flung into space? It would just keep on merrily going around where its sun used to be until it "learned" that the sun was gone and decided to leave its solar system? Wouldn't a Pluto-distance planet just keep going for 4 hours, and then depart? That seems weird to me, but it appears to follow from what they're saying.
 
2012-09-13 06:38:52 AM  
Thanks for all your interest in my appearance on NPR's RadioLab segment on "levitating" Slinkies. Based on some comments in this thread, I am compelled to offer a few more clarifying points on what's going on. The Slinky's bottom is not the only part that does not move when the top is released. **Every coil in the Slinky** remains motionless until the top falls to greet it. So the argument that the Slinky's bottom recoils upwards at the same rate that the Slinky falls is a compelling but false account. Not part of a Slinky can react to what any other part of a Slinky is doing sooner than the wave travel speed for that Slinky allows it. (The wave travel speed is how fast a push-pull pulse moves along the length of the toy.) For a dropped Slinky, with the help of gravity, the top falls faster than the Slinky's native wave speed can propagate. This means no coil of the Slinky has any clue -- at any time -- that the top was released, until the falling top greets it head-on. It's this fact that generates the spooky image of the Slinky suspended in mid-air.

Neil deGrasse Tyson
New York City
 
2012-09-13 12:41:26 PM  

neiltyson: Thanks for all your interest in my appearance on NPR's RadioLab segment on "levitating" Slinkies. Based on some comments in this thread, I am compelled to offer a few more clarifying points on what's going on. The Slinky's bottom is not the only part that does not move when the top is released. **Every coil in the Slinky** remains motionless until the top falls to greet it. So the argument that the Slinky's bottom recoils upwards at the same rate that the Slinky falls is a compelling but false account. Not part of a Slinky can react to what any other part of a Slinky is doing sooner than the wave travel speed for that Slinky allows it. (The wave travel speed is how fast a push-pull pulse moves along the length of the toy.) For a dropped Slinky, with the help of gravity, the top falls faster than the Slinky's native wave speed can propagate. This means no coil of the Slinky has any clue -- at any time -- that the top was released, until the falling top greets it head-on. It's this fact that generates the spooky image of the Slinky suspended in mid-air.


Woah. We're dealing with a badass here.
 
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